Computing forwarding tables for link failures

ABSTRACT

A method for maintaining a bridging network communication path table is presented and includes determining a failed communication link between a first bridge computer and a second bridge computer in the bridging network; identifying, in the communication path table, a first path that includes the failed communication link; and indicating that the first path is not active.

FIELD OF THE INVENTION

The present invention relates generally to the field of routing data ina network, and more particularly to link failure response in bridgingnetworks.

BACKGROUND OF THE INVENTION

The data link layer, layer 2 of the seven-layer OSI (open systemsinterconnection) model of computer networking, is the protocol layerthat transfers data between adjacent network nodes in a wide areanetwork or between nodes on the same local area network segment. Thedata link layer provides the functional and procedural means to transferdata between network entities.

Unicast transmission, of unicast traffic, is the sending of data-linkframes from a single source to a single network destination identifiedby a unique IP (internet protocol) address. Multi-destinationtransmission is the sending of data-link frames in a single transmissionfrom a single source simultaneously to either: (i) a group ofdestination computers (multicast); or (ii) every device on the network(broadcast).

The spanning tree protocol (STP) is a network protocol that ensures aloop-free topology for any bridged ethernet local area network (LAN).The basic function of STP is to prevent bridge loops and the broadcastradiation that results from them. The spanning tree also allows anetwork design to include spare, or redundant, links to provideautomatic backup paths if an active link fails, without the danger ofbridge loops, or the need for manual enabling/disabling of these backuplinks. STP creates a spanning tree within a network of connected layer-2bridges (typically ethernet switches), and disables those links that arenot part of the spanning tree, leaving a single active path between anytwo network nodes.

TRILL (transparent interconnection of lots of links) is an internetengineering task force (IETF) standard for enabling multipathing in adata center. The TRILL campus topology is implemented by devices calledrouting bridges (RBridges) or TRILL Switches. Routing bridges run a linkstate protocol amongst themselves. A link state protocol is one in whichconnectivity is broadcast to all the RBridges, so that each RBridgeknows about all the other RBridges, and the connectivity between them.This gives RBridges enough information to calculate shortest path first(SPF) paths where an equal cost multipath (ECMP) routing strategy isavailable for unicast traffic, and calculate distribution trees fordelivery of frames either to destinations whose location is unknown orto multi-destination groups.

A TRILL campus topology may support multiple topologies such that bothunicast and multi-destination traffic is routed in the network. In amulti-topology network, the router uses logically different routingtables for different topologies.

ECMP is a routing strategy where next-hop frame, or packet, forwardingto a single destination can occur over multiple equal cost paths, orroutes. Equal cost paths are those paths, or routes, that tie for topplace in routing metric calculations.

The control plane is the part of the router architecture that isconcerned with drawing the network map, or the information in a routingtable that defines what to do with incoming frames. In most cases, therouting table contains a list of destination addresses and the outgoingnode(s) associated with them.

The forwarding plane, sometimes called the data plane, defines the partof the router architecture that decides what to do with frames arrivingon an inbound node. Most commonly, it refers to a routing table in whichthe router looks up the destination address of the incoming frame andretrieves the information necessary to determine the route from thereceiving node, through the internal forwarding fabric of the router,and to the proper outgoing node(s).

SUMMARY

A method for maintaining a bridging network communication path table ispresented including: determining a failed communication link between afirst bridge computer and a second bridge computer in the bridgingnetwork; identifying, in the communication path table, a first path thatincludes the failed communication link; and indicating that the firstpath is not active.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic view of a first embodiment of a networkedcomputers system according to the present invention;

FIG. 2 is a schematic view of a portion of the first embodimentnetworked computers system;

FIG. 3 is a flowchart showing a method performed, at least in part, bythe first embodiment computer system;

FIG. 4A is a first ECMP topology for unicast traffic, in a no brokenlink status, according to an embodiment of the present invention;

FIG. 4B is the first ECMP topology for unicast traffic, in a broken linkstatus, according to an embodiment of the present invention;

FIG. 5A is a second ECMP topology for multi-destination traffic, in a nobroken link status, according to an embodiment of the present invention;and

FIG. 5B is the second ECMP topology for multi-destination traffic, in abroken link status, according to an embodiment of the present invention.

DETAILED DESCRIPTION

This Detailed Description section is divided into the followingsub-sections: (i) The Hardware and Software Environment; (ii) FirstEmbodiment; (iii) Further Comments and/or Embodiments; and (iv)Definitions.

I. THE HARDWARE AND SOFTWARE ENVIRONMENT

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer-readablemedium(s) having computer readable program code/instructions embodiedthereon.

Any combination of computer-readable media may be utilized.Computer-readable media may be a computer-readable signal medium or acomputer-readable storage medium. A computer-readable storage medium maybe, for example, but not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus, ordevice, or any suitable combination of the foregoing. More specificexamples (a non-exhaustive list) of a computer-readable storage mediumwould include the following: an electrical connection having one or morewires, a portable computer diskette, a hard disk, a random access memory(RAM), a read-only memory (ROM), an erasable programmable read-onlymemory (EPROM or Flash memory), an optical fiber, a portable compactdisc read-only memory (CD-ROM), an optical storage device, a magneticstorage device, or any suitable combination of the foregoing. In thecontext of this document, a computer-readable storage medium may be anytangible medium that can contain, or store a program for use by or inconnection with an instruction execution system, apparatus, or device.

A computer-readable signal medium may include a propagated data signalwith computer-readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer-readable signal medium may be any computer-readable medium thatis not a computer-readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer-readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java (note: the term(s) “Java” may be subject to trademarkrights in various jurisdictions throughout the world and are used hereonly in reference to the products or services properly denominated bythe marks to the extent that such trademark rights may exist),Smalltalk, C++ or the like and conventional procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The program code may execute entirely on a user's computer,partly on the user's computer, as a stand-alone software package, partlyon the user's computer and partly on a remote computer or entirely onthe remote computer or server. In the latter scenario, the remotecomputer may be connected to the user's computer through any type ofnetwork, including a local area network (LAN) or a wide area network(WAN), or the connection may be made to an external computer (forexample, through the Internet using an Internet Service Provider).

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in acomputer-readable medium that can direct a computer, other programmabledata processing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer-readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce acomputer-implemented process such that the instructions which execute onthe computer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

An embodiment of a hardware and software environment in which thepresent invention may be utilized will now be described in detail withreference to the Figures. FIGS. 1 and 2 collectively make up afunctional block diagram illustrating various portions of distributeddata processing system 100, including: bridging network 101; bridgesub-systems 102, 104, 106, 108, 110, 112; first portion 114 a of localarea network (LAN) 114; second portion 114 b of LAN 114; usersub-systems 118, 120, 122, 124, 126; first bridge computer 200;communication unit 202; processor set 204; input/output (i/o) interfaceset 206; memory device 208; persistent storage device 210; displaydevice 212; external device set 214; random access memory (RAM) device230; cache memory device 232; program 240; algorithm module 250; table255; change table module 260; send communication module 265; andmaintenance module 270.

As shown in FIG. 2, bridge sub-system 102 is built and programmed tocommunicate data (for example, packetized data) between portions of LAN114. For this reason, bridge computer 200 is a special purpose computerbuilt and programmed specifically for communicating quickly andefficiently, as will be understood by those of skill in the data networkcommunication design art. Bridging network 101 may further includeswitch computers (not shown). Program 240 is a is a collection ofmachine readable instructions and/or data that is used to create, manageand control network communication bridge network related functions thatwill be discussed in detail, below, in the First Embodiment sub-sectionof this Detailed Description section.

Bridge sub-system 102 is capable of communicating with other bridgesub-systems via bridging network 101 (see FIG. 1). Bridging network 101can be, for example, a local area network (LAN), a wide area network(WAN) such as the Internet, or a combination of the two, and can includewired, wireless, or fiber optic connections. In general, bridgingnetwork 101 can be any combination of connections and protocols thatwill support communications between bridge and user sub-systems.

It should be appreciated that FIGS. 1 and 2, taken together, provideonly an illustration of one implementation (that is, system 100) anddoes not imply any limitations with regard to the specific networkarchitecture of the various environments in which different embodimentsmay be implemented. Many modifications to the depicted environment maybe made, especially with respect to current and anticipated futureadvances in cloud computing, distributed computing, smaller computingdevices, network communications and the like.

As shown in FIG. 2, bridge sub-system 102 is shown as a block diagramwith many double arrows. These double arrows (no separate referencenumerals) represent a communications fabric, which providescommunications between various components of sub-system 102. Thiscommunications fabric can be implemented with any architecture designedfor passing data and/or control information between processors (such asmicroprocessors, communications and network processors, etc.), systemmemory, peripheral devices, and any other hardware components within asystem. For example, the communications fabric can be implemented, atleast in part, with one or more buses.

Memory 208 and persistent storage 210 are computer-readable storagemedia. In general, memory 208 can include any suitable volatile ornon-volatile computer-readable storage media. It is further noted that,now and/or in the near future: (i) external device(s) 214 may be able tosupply, some or all, memory for sub-system 102; and/or (ii) devicesexternal to sub-system 102 may be able to provide memory for sub-system102.

Program 240 is stored in persistent storage 210 for access and/orexecution by one or more of the respective computer processors 204,usually through one or more memories of memory 208. Persistent storage210: (i) is at least more persistent than a signal in transit; (ii)stores the device on a tangible medium (such as magnetic or opticaldomains); and (iii) is substantially less persistent than permanentstorage. Alternatively, data storage may be more persistent and/orpermanent than the type of storage provided by persistent storage 210.

Program 240 may include both machine readable and performableinstructions and/or substantive data (that is, the type of data storedin a database). In this particular embodiment, persistent storage 210includes a magnetic hard disk drive. To name some possible variations,persistent storage 210 may include a solid state hard drive, asemiconductor storage device, read-only memory (ROM), erasableprogrammable read-only memory (EPROM), flash memory, or any othercomputer-readable storage media that is capable of storing programinstructions or digital information. The various modules and/or datablocks 250, 255, 260, 265, 270 will be discussed in detail below, in theFirst Embodiment sub-section of this Detailed Description section.

The media used by persistent storage 210 may also be removable. Forexample, a removable hard drive may be used for persistent storage 210.Other examples include optical and magnetic disks, thumb drives, andsmart cards that are inserted into a drive for transfer onto anothercomputer-readable storage medium that is also part of persistent storage210.

Communications unit 202, in these examples, provides for communicationswith other data processing systems or devices external to sub-system102, such as bridge sub-systems 104, 106, 108, 110, 112 and LAN network114 (shown as two portions, 114 a and 114 b). In these examples,communications unit 202 includes one or more network interface cards.Communications unit 202 may provide communications through the use ofeither or both physical and wireless communications links. Any softwaremodules discussed herein may be downloaded to a persistent storagedevice (such as persistent storage device 210) through a communicationsunit (such as communications unit 202).

I/O interface set 206 allows for input and output of data with otherdevices that may be connected locally in data communication with firstbridge computer 200. For example, I/O interface set 206 provides aconnection to external device set 214. External device set 214 willtypically include devices such as a keyboard, keypad, touch screen,and/or some other suitable input device. External device set 214 canalso include portable computer-readable storage media such as, forexample, thumb drives, portable optical or magnetic disks, and memorycards. Software and data used to practice embodiments of the presentinvention, for example, program 240, can be stored on such portablecomputer-readable storage media. In these embodiments the relevantsoftware may (or may not) be loaded, in whole or in part, ontopersistent storage device 210 via I/O interface set 206. I/O interfaceset 206 also connects in data communication with display device 212.

Display device 212 provides a mechanism to display data to a user andmay be, for example, a computer monitor or a smart phone display screen.

The programs described herein are identified based upon the applicationfor which they are implemented in a specific embodiment of theinvention. However, it should be appreciated that any particular programnomenclature herein is used merely for convenience, and thus theinvention should not be limited to use solely in any specificapplication identified and/or implied by such nomenclature.

II. FIRST EMBODIMENT

Preliminary note: The flowchart and block diagrams in the followingFigures illustrate the architecture, functionality, and operation ofpossible implementations of systems, methods and computer programproducts according to various embodiments of the present invention. Inthis regard, each block in the flowchart or block diagrams may representa module, segment, or portion of code, which comprises one or moreexecutable instructions for implementing the specified logicalfunction(s). It should also be noted that, in some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

FIG. 2 shows program 240 for performing at least some of the methodsteps of flow chart 300. FIG. 3 shows a flow chart 300 depicting amethod according to the present invention. This software and associatedmethod will now be discussed, over the course of the followingparagraphs, with extensive reference to FIG. 2 (for the software blocks)and FIG. 3 (for the method step blocks).

Processing begins at step S305, where network set-up technicians (notshown) set up bridge network 101 (see FIG. 1) with multiple networkedbridge sub-systems 102, 104, 106, 108, 110, and 112 (see FIG. 1). Abridge network transfers communication traffic, or, simply, data, fromone network to another network via various communication paths made upof one or more communication links between the individual bridgesub-systems making up the bridge network. In this embodiment, bridgenetwork 101 transfers data from first portion 114 a of LAN 114 to secondportion 114 b of LAN 114 (see FIG. 1). In some embodiments of thepresent disclosure, the bridge network includes conventional routingbridges in a TRILL campus topology. TRILL-based embodiments arediscussed in more detail below in the Further Comments and/orEmbodiments Section of this Detailed Description.

Processing proceeds to step S310, where algorithm module 250 applies ashortest path first (SPF) algorithm to determine a first communicationpath table 255 for first bridge sub-system 102. In this embodiment, eachbridge sub-system has a corresponding communication path table (notshown for bridge sub-systems 104, 106, 108, 110, and 112).Alternatively, the control plane in the router architecture (not shown)has a bridge network communication path table (not shown) describing thelinks and corresponding paths throughout the bridge network. As afurther alternative, a different algorithm (now known or to be developedin the future), other than SPF, could be used to determine the variouspaths stored in table 255. However, even under a different algorithm,these various paths will: (i) be made up of links (which are susceptibleto breakage); and (ii) have some order of relative preference.

Each path determined by the SPF algorithm is assigned valuesrepresenting one or more path characteristics. In this embodiment, a“link” value is assigned to each communication path. The “link” value isthe number of links that make up a single communication path.Alternatively, the path determination is based on path characteristicswithin a given communication path including, but not limited to: (i) thereliability of the links; (ii) the status of the links; and/or (iii) thevolume of traffic expected to be transferred through the links. Table 1,shown below, illustrates one embodiment of first communication pathtable 255 determined by algorithm mod 250 for first bridge sub-system102.

TABLE 1 FIRST COMMUNICATION PATH TABLE: INITIAL STATUS. NO. ofDESTINATION PATH LINKS ACTIVE 2^(ND) BRIDGE 1-2 1 Y 2^(ND) BRIDGE 1-4;4-2 2 Y 2^(ND) BRIDGE 1-3; 3-4; 4-2 3 Y . . . . . . . . . . . . 6^(TH)BRIDGE 1-4; 4-3; 3-5; 5-6 4 Y 6^(TH) BRIDGE 1-2; 2-4; 4-3; 3-5; 5-6 5 Y

Note how each of the five paths shown in (abbreviated) Table 1correspond to the link lines shown within network 101 in FIG. 1. Furthernote that each path only passes through a given bridge sub-system 104,106, 108, 110, 112 once on its way to its path destination. In the abovefirst communication path table, the link value of each communicationpath is recorded. As mentioned above, the link value refers to thenumber of links in a given communication path. There are threecommunication paths shown in Table 1 that reach second bridge 104. Thepaths in Table 1 are denoted by the one or more communication links thatmake up each path. For example, a link may transfer data from firstbridge sub-system 102 to second bridge sub-system 104. The examplecommunication link is written “1-2,” where the first number in thenumber pair is the originating bridge sub-system number and the secondnumber is the destination bridge sub-system number. Each of the links ina path is listed sequentially by corresponding number pair and separatedby a semi-colon. One path in Table 1 is made up of a single link, 1-2,with a corresponding link value of 1. There is also shown acommunication path made up of three links, 1-3; 3-4; 4-2, with acorresponding link value of 3.

The communication path table includes an indication as to whether thecorresponding path is active or inactive according to the column labeled“active.” In this example, each of the shown paths is active. It shouldbe understood that additional or alternative path characteristics may betracked in a communication path table.

Processing proceeds to step S315, where change table module 260determines that a communication link in bridge network 101 has failed.When a communication link fails, the data traffic is unable to transferbetween the two bridge sub-systems that communicate through the failedlink. For example, when the communication link between first bridge 102and second bridge 104 (shown in FIG. 1) fails, the change table moduledetermines that a failure has occurred. Alternatively, a control planein the router architecture (not shown) determines that a communicationlink has failed, or is down.

Processing proceeds to step S320, where change table module 260 changescommunication path tables in the bridge network to indicate that anypaths including the failed link are inactive. In Table 2, showinganother embodiment of first communication path table 255, the failedpaths are indicated by setting the status of the path to inactive (shownby an “N” in the active status column).

TABLE 2 FIRST COMMUNICATION PATH TABLE: CHANGED AT STEP S320. NO. ofDESTINATION PATH LINKS ACTIVE 2ND BRIDGE 1-2 1 N 2ND BRIDGE 1-4; 4-2 2 Y2ND BRIDGE 1-3; 3-4; 4-2 3 Y . . . . . . . . . . . . 6TH BRIDGE 1-4;4-3; 3-5; 5-6 4 Y 6TH BRIDGE 1-2; 2-4; 4-3; 3-5; 5-6 5 N

Continuing with the example above, where the communication link betweenbridge sub-system 102 and bridge sub-system 104, or link 1-2, hasfailed, change table mod 260 changes the first communication path tablefrom the one shown in Table 1 to the communication path table shown inTable 2. Table 2 includes two paths that use the communication link 1-2.Those paths that use link 1-2 are shown in Table 2 as inactive, that is,the active column is marked “N.”

Processing proceeds to step S325, where send communication module 265transfers communication data from first bridge sub-system 102 over acommunication path that does not include inactive communication link1-2. As can be seen in Table 2, there is more than one activecommunication path through which first bridge sub-system 102 can reachsecond bridge sub-system 104. In this embodiment, the new communicationpath is the active path having the least cost. Alternatively, the newcommunication path is one that meets another specified pathcharacteristic(s).

Processing proceeds to step S330, where maintenance module 270re-applies the SPF algorithm to re-determine the first communicationpath table 255 and the corresponding tables of bridge sub-systems 104,106, 108, 110, and 112. This recalculation in done at relatively largetime intervals in the normal course of maintenance. This means that stepS325 may be repeated a great number of times before step S330 isperformed. When communication path tables are computed by themaintenance mod, change table mod 260 changes the tables according tothe re-determination. By using the inactive designations of table 255 atstep S325, this saves bridge computer 200, and/or other systemprocessing resources, from the time and/or processing burden ofrecalculating the entirety of table 255 every time a link breaks in thebridging network.

III. FURTHER COMMENTS AND/OR EMBODIMENTS

Some embodiments of the present disclosure recognize that, in TRILL: (i)every link up or link down indication triggers an SPF computation; (ii)this SPF computation creates new forwarding tables to route traffic;(iii) the SPF computation usually is the most CPU intensive operation inTRILL; and (iv) it is, therefore, desirable to minimize the number ofSPF computations.

Some embodiments of the present disclosure may include one, or more, ofthe following features, characteristics, and/or advantages: (i) certainactions where ECMP routing strategy is used; (ii) optimizations suchthat new forwarding tables can be computed without triggeringTRILL-based SPF computation; and/or (iii) providing for rapid re-routingof network traffic in response to link failures.

Some embodiments of the present disclosure detect, for every linkfailure, if the failed link is part of: (i) a unicast SPF ECMP topology;or (ii) a multi-destination ECMP topology. Where the failed link is partof either of the two ECMP topologies noted above, the respective unicastSPF computation or multi-destination tree creation is not triggered.

Some embodiments of the present disclosure may or may not have one, ormore, of the following features, characteristics, and/or advantages: (i)construction of new forwarding tables without triggering expensive SPFcomputation; (ii) control plane workload reduction by not requiring animmediate SPF computation; (iii) rapid response to link failuresresulting in fast re-route of network traffic; and/or (iv) minimizesnetwork traffic disruptions.

FIG. 4A depicts a first unicast SPF ECMP topology 400 according to anembodiment of the present invention. Unicast ECMP topology 400 includes:routing bridge, or node, RB1; routing bridge RB2; routing bridge RB3;routing bridge RB4; links 402, 404, 406, and 408.

For unicast TRILL route tables, there are two ECMP paths from RB1 toRB4. The first ECMP path is made up of links 404 and 406. The secondECMP path is made up of links 402 and 408. Each of the two ECMP pathsare “equal cost” paths in that the sums of the costs of the individuallinks 404, 406 and 402, 408 within each path are equal. For example, thecost of each path may be five. For the first EMCP path, link 404 mayhave an associated cost of 3 and link 406 may have a cost of 2, whilethe second ECMP path includes link 402, with a cost of 1, and link 408,with a cost of 4.

Both ECMP paths are stored in routing bridge RB1. FIG. 4B depictsunicast SPF ECMP topology 420 according to an embodiment of the presentinvention. In FIG. 4B, ECMP link 406 is not shown (see FIG. 4A) becauseit is down. RB1 detects that one of the two ECMP links to RB4 is down byconsulting its routing table. Because there are two ECMP paths stored inRB1's routing table, when 406 is down, the alternative path of links 402and 408 is programmed (in a routing table in RB1) as the path to beused. It is because the routing bridge retains a record of the alternateECMP path that the control plane is able to push the alternate path whenthe active path goes down. By pushing the alternative path, an SPFcomputation is avoided, or, at least, postponed until routinemaintenance occurs. The broken ECMP link 406 may be deleted from thehardware and software tables without re-computing the SPF.Alternatively, the broken link continues to be stored in the routingbridge for later use when the broken link comes back up.

FIG. 5A depicts a first multi-destination ECMP topology, ormulti-destination tree, according to an embodiment of the presentinvention. Multi-destination tree 440 is a loop free tree including:highest priority tree root RB5; nodes RB6, RB7, and RB8; parent pointers442, 444; parent pointer set 446 a, 446 b; and links 448, 450, 452.

Where RB5 is the highest priority tree root, the multi-destination treeis created with each other node have parent pointer(s) toward root RB5.At each node, the parent pointers are maintained such that: (i) RB6includes parent pointer 442 directed to RB5; (ii) RB73 includes parentpointer 444 directed to RB5; and (iii) RB8 has two parent pointersdirected to intermediate nodes that reach root RB5. Parent pointer 446 ais directed to RB6 and parent pointer 446 b is directed to RB7. However,only link 452, directed by parent pointer 446 a, is used for the loopfree tree according to FIG. 5A.

FIG. 5B depicts a second multi-destination ECMP topology, ormulti-destination tree, according to an embodiment of the presentinvention. Multi-destination tree 460 is a loop free tree including:highest priority tree root RB5; nodes RB6, RB7, and RB8; parent pointers442, 444; parent pointer set 446 a, 446 b; and routes 448, 450, 462. InFIG. 5B link 452 is not shown (see FIG. 5A). RB5 detects that the linkdown communicates with destination node, RB8, which has two parentpointers. Because there are two parent pointers, RB5 reprograms itsrouting table to use parent pointer 446 a, which is directed to RB6 overactive link 452. In this example, new multi-destination tree 460 isquickly constructed using the stored parent pointer 446 b without theneed to trigger an immediate SPF computation to create a new tree.

Some embodiments of the present disclosure recognize that TRILL isdifferent from many other protocols in that it utilizes tables for bothunicast and multi-destination frames. ECMP detection is different ineach set, or protocol. Recovery from a link up/down condition isdifferent with TRILL than other link-state routing protocols such asOSPF (open shortest path first), in that OSPF only computes unicastpaths.

Some embodiments of the present disclosure avoid the high cost ofperforming the SPF computation at each status change by recording for afirst node each ECMP path from the first node to a next node for use bythe control plane. In that way, the control plane's response time foralternative link detection is faster because there is no need to computethe SPF for the ECMP paths.

Some embodiments of the present disclosure record the fact that some ofthe paths in a network are ECMP paths and there is the ability to reachfrom a first node to a second node through multiple paths when theshortest path graph is computed. Typically, in the TRILL topology, onepath is normally set as the active path.

Some embodiments of the present disclosure, record the fact that thereare multiple paths to reach from, for example, Node A to Node B even ifthe alternative communication paths are not active. In this way, whenone link goes down, a replacement link is quickly identified andinstalled into the forwarding database of the corresponding node. Theinformation regarding the failed link may be applied to the TRILLtopology, in a periodic time, during regular maintenance. At that time,an SPF graph is re-computed for the current system. At times other thanroutine maintenance, a running change is made possible in response to afailed link by recording the alternative ECMP paths that were notselected during previous SPF computations.

Some embodiments of the present disclosure use the SPF graph informationto recognize the paths having ECMP links. One of the ECMP linksidentified in the SPF computation is typically selected using logicincluding, but not limited to: (i) a hashing process; and/or (ii) thehighest assigned number. When the chosen ECMP link goes down, or fails,the routing bridge is able to quickly program the hardware with a newECMP link that is available.

Some embodiments of the present disclosure apply to network traffic thatis either: (i) multi-destination; or (ii) unicast. Multi-destination, asthe name suggests, is a communication that is not directed to anindividual destination, but to multiple nodes, or RBridges. The onlydifference between unicast and multi-destination is that there are twokinds of topology calculations. For the multi-destination calculation,even in the hardware, there is no way to actually look at multiplepaths, because only one path is selected. For additional paths in amulti-destination communication, there must be another tree. For unicastcommunications, there is an ability to actually use multiple paths, butthe ECMP link options still have to be tracked to avoid an SPFre-calculation when the active ECMP link fails.

Some embodiments of the present disclosure include two steps: (i) quickdetection of whether or not the failed path is due to a failed ECMPlink; and (ii) once detected, how to actually use the information toprogram it to avoid the resource demanding SPF computation.

Some embodiments of the present disclosure detect an ECMP path byreference to pointers. When a failure occurs, a determination is made asto whether there are any pointers, which are associated withmulti-destination traffic. If the number of parent pointers is greaterthan one, then there are at least two parents, so the corresponding linkmust be part of a multi-destination tree.

Some embodiments of the present disclosure use a method including thesteps that follow: (i) create an SPF graph; (ii) reach steady stateoperations; (iii) build a topology; (iv) record the topology; and (v)when any state change notice is received from the network, a decisionregarding whether or not to run an SPF computation is made; (vi)determine if there is a faster way to determine an alternative link; and(vii) program the hardware.

IV. DEFINITIONS

Present invention: should not be taken as an absolute indication thatthe subject matter described by the term “present invention” is coveredby either the claims as they are filed, or by the claims that mayeventually issue after patent prosecution; while the term “presentinvention” is used to help the reader to get a general feel for whichdisclosures herein that are believed as maybe being new, thisunderstanding, as indicated by use of the term “present invention,” istentative and provisional and subject to change over the course ofpatent prosecution as relevant information is developed and as theclaims are potentially amended.

Embodiment: see definition of “present invention” above—similar cautionsapply to the term “embodiment.”

and/or: inclusive or; for example, A, B “and/or” C means that at leastone of A or B or C is true and applicable.

User/subscriber: includes, but is not necessarily limited to, thefollowing: (i) a single individual human; (ii) an artificialintelligence entity with sufficient intelligence to act as a user orsubscriber; and/or (iii) a group of related users or subscribers.

Electrically Connected: means either directly electrically connected, orindirectly electrically connected, such that intervening elements arepresent; in an indirect electrical connection, the intervening elementsmay include inductors and/or transformers.

Data communication: any sort of data communication scheme now known orto be developed in the future, including wireless communication, wiredcommunication and communication routes that have wireless and wiredportions; data communication is not necessarily limited to: (i) directdata communication; (ii) indirect data communication; and/or (iii) datacommunication where the format, packetization status, medium, encryptionstatus and/or protocol remains constant over the entire course of thedata communication.

Receive/provide/send/input/output: unless otherwise explicitlyspecified, these words should not be taken to imply: (i) any particulardegree of directness with respect to the relationship between theirobjects and subjects; and/or (ii) absence of intermediate components,actions and/or things interposed between their objects and subjects.

Module/Sub-Module: any set of hardware, firmware and/or software thatoperatively works to do some kind of function, without regard to whetherthe module is: (i) in a single local proximity; (ii) distributed over awide area; (ii) in a single proximity within a larger piece of softwarecode; (iii) located within a single piece of software code; (iv) locatedin a single storage device, memory or medium; (v) mechanicallyconnected; (vi) electrically connected; and/or (vii) connected in datacommunication.

Software storage device: any device (or set of devices) capable ofstoring computer code in a manner less transient than a signal intransit.

Tangible medium software storage device: any software storage device(see Definition, above) that stores the computer code in and/or on atangible medium.

Non-transitory software storage device: any software storage device (seeDefinition, above) that stores the computer code in a non-transitorymanner.

Computer: any device with significant data processing and/or machinereadable instruction reading capabilities including, but not limited to:desktop computers, mainframe computers, laptop computers,field-programmable gate array (fpga) based devices, smart phones,personal digital assistants (PDAs), body-mounted or inserted computers,embedded device style computers, application-specific integrated circuit(ASIC) based devices.

What is claimed is:
 1. A method for maintaining a bridging networkcommunication path table, the method comprising: performing a shortestpath first (SPF) computation to build a communication path table for abridging network that has a transparent interconnection of lots of links(TRILL) topology with equal cost multipath (ECMP) paths, wherein thecommunication path table indicates a plurality of active ECMP pathsbetween a first routing bridge and a second routing bridge in thebridging network, and wherein the SPF computation ranks each active ECMPpath of the plurality of active ECMP paths based on: a link value thatrepresents a number of links included in the active ECMP path, areliability value for one or more of the links included in the activeECMP path, and an expected volume of traffic to be transferred throughone or more of the links included in the active ECMP path; determining afailed communication link between the first routing bridge and thesecond routing bridge in the bridging network; identifying, in thecommunication path table, a first ECMP path of the plurality of activeECMP paths that includes the failed communication link; and responsiveto identifying the first ECMP path that includes the failedcommunication link, adding an indication to the communication path tablethat the first ECMP path is not active and postponing an SPF computationto rebuild the communication path table until a later time.
 2. Themethod of claim 1 wherein the communication path table is stored at athird routing bridge, and the communication path table indicates pathsoriginating at the third routing bridge.
 3. The method of claim 1wherein the communication path table is for one of: unicast traffic ormulti-destination traffic.
 4. A computer program product for maintaininga bridging network communication path table, the computer programproduct comprising program instructions stored on a computer-readablehardware storage device, the stored program instructions comprising:program instructions to perform a shortest path first (SPF) computationto build a communication path table for a bridging network that has atransparent interconnection of lots of links (TRILL) topology with equalcost multipath (ECMP) paths, wherein the communication path tableindicates a plurality of active ECMP paths between a first routingbridge and a second routing bridge in the bridging network, and whereinthe SPF computation ranks each active ECMP path of the plurality ofactive ECMP paths based on: a link value that represents a number oflinks included in the active ECMP path, a reliability value for one ormore of the links included in the active ECMP path, and an expectedvolume of traffic to be transferred through one or more of the linksincluded in the active ECMP path; program instructions to determine afailed communication link between the first routing bridge and thesecond routing bridge in the bridging network; program instructions toidentify, in the communication path table, a first ECMP path of theplurality of active ECMP paths that includes the failed communicationlink; and program instructions to, responsive to identifying the firstECMP path that includes the failed communication link, add an indicationto the communication path table that the first ECMP path is not activeand postpone an SPF computation to rebuild the communication path tableuntil a later time.
 5. The computer program product of claim 4 whereinthe communication path table is stored at a third routing bridge, andthe communication path table indicates paths originating at the thirdrouting bridge.
 6. The computer program product of claim 4 wherein thecommunication path table is for one of: unicast traffic ormulti-destination traffic.
 7. A computer system for maintaining abridging network communication path table, the computer systemcomprising: a processor(s) set; and a computer-readable hardware storagedevice; and program instructions stored on the computer-readablehardware storage device for execution by the processor(s) set, thestored program instructions comprising: program instructions to performa shortest path first (SPF) computation to build a communication pathtable for a bridging network that has a transparent interconnection oflots of links (TRILL) topology with equal cost multipath (ECMP) paths,wherein the communication path table indicates a plurality of activeECMP paths between a first routing bridge and a second routing bridge inthe bridging network, and wherein the SPF computation ranks each activeECMP path of the plurality of active ECMP paths based on: a link valuethat represents a number of links included in the active ECMP path, areliability value for one or more of the links included in the activeECMP path, and an expected volume of traffic to be transferred throughone or more of the links included in the active ECMP path; programinstructions to determine a failed communication link between the firstrouting bridge and the second routing bridge in the bridging network;program instructions to identify, in the communication path table, afirst ECMP path of the plurality of active ECMP paths that includes thefailed communication link; and program instructions to, responsive toidentifying the first ECMP path that includes the failed communicationlink, add an indication to the communication path table that the firstECMP path is not active and postpone an SPF computation to rebuild thecommunication path table until a later time.
 8. The computer system ofclaim 7 wherein the communication path table is stored at a thirdrouting bridge, and the communication path table indicates pathsoriginating at the third routing bridge.
 9. The computer system of claim7 wherein the communication path table is for one of: unicast traffic ormulti-destination traffic.
 10. The method of claim 1, furthercomprising: identifying a second ECMP path of the plurality of activepaths in the communication path table; and re-routing network trafficbetween the first routing bridge and the second routing bridge using thesecond ECMP path.
 11. The method of claim 1, further comprising:responsive to determining that the failed communication link between thefirst routing bridge and the second routing bridge has been restored,indicating that the first ECMP path is active.
 12. The method of claim1, wherein the SPF computation is postponed until a routine maintenanceperiod occurs.